1. Field of the Invention
The present invention relates to a continuously variable transmission installed in a vehicle, such as an automobile. In particular, the present invention relates to a continuously variable transmission capable of selecting a gear ratio in accordance with an operation of a driver.
2. Description of the Related Art
As a transmission that changes the speed of an output of a power source (used for travel) of, for example, an engine of an automobile, a continuously variable transmission having a good fuel consumption rate during actual travel is increasingly used. For such a continuously variable transmission, a type provided with a so-called manual mode, in which any one gear ratio is selected from a plurality of gear ratios previously set in accordance with a gear-change operation of a driver, is proposed, in addition to a type that performs an automatic gear-change operation in which a gear ratio is automatically set in accordance with a request for a torque from a driver or a travel state of a vehicle.
In the manual mode, in general, gear ratios corresponding to, for example, approximately five to seven gears are previously set, and a gear ratio is successively changed to a gear ratio corresponding to a next gear in accordance with an upshift operation and a downshift operation of a driver.
Hitherto, as a related technology related to a continuously variable transmission provided with such a manual mode, for example, Japanese Unexamined Patent Application Publication No. 2005-195158 discusses a continuously variable transmission which, when a minimum value of an input number of rotations is set in accordance with a travel resistance of a vehicle and when a gear-change operation that causes the input number of rotations to become less than the minimum value is continuously performed, accepts such a gear-change operation. In addition, Japanese Unexamined Patent Application Publication No. 2005-201320 discusses a continuously variable transmission provided with a manual mode that, when a request is made for a braking operation without any gear-change operation, controls a gear ratio so that a predetermined input number of rotations is maintained, and that, when a request for a braking operation is less frequently made, maintains a current gear ratio.
In an existing continuously variable transmission provided with a manual mode, there exist, for example, a case in which when one gear ratio is selected from gear ratios corresponding to, for example, approximately five to seven gears, a suitable gear ratio cannot always be properly set in accordance with various travel states.
For example, when, for the purpose of providing a proper rising acceleration at a corner and of increasing a braking force of an engine brake, downshifting is performed for increasing the rotation speed of the engine, the rotation speed of the engine may not be sufficiently increased by downshifting by only one gear; and the rotation speed of the engine may become too high by downshifting by two gears.